In gas tungsten arc welding, sometimes referred to as TIG welding, the output current can be provided by any number of constant current power sources. Most metals are capable of being welded by the GTAW or TIG welding process; however, they are generally welded by a DC negative arc using an inert gas atmosphere. When welding aluminum or magnesium using the TIG welding process, the current is generally AC with an electrode positive current section and an electrode negative current section. These two sections are balanced using early technology; however, more recently, variable polarity power sources have become available. Some of these AC power sources can make only a limited amount of change between the positive polarity current section and the negative polarity current section. Using inverter technology with a waveform generator, as pioneered by assignee of the present invention, the positive current section and the negative current section of the waveform for TIG welding can be adjusted in shape, amplitude and duration. The normal DC TIG welding is performed with an alloyed tungsten electrode, whereas AC TIG welding of the fixed waveform type or the variable waveform type is performed with a pure tungsten electrode. The size of the electrode is determined by the application and the current range required for the welding process. If a given size electrode is welded at a current above its desired range of operation, the tungsten electrode may become too hot and the electrode will “spit” metal into the welding pool. This is not acceptable. In most cases the electrode spitting occurs in the positive half cycle or section of the waveform when the instantaneous positive current is too high for the particular electrode being used. Thus, when AC TIG welding, the magnitude and/or duration of the different current sections are controlled for adjusting the welding process to the desired conditions. DC TIG welding processes have been improved by using a pulsed technology. DC positive or DC negative TIG welding is now performed using a pulsed waveform since welders now have the capabilities of producing successive high and low current sections. Indeed, pulsing of the positive current or the negative current in TIG welding has become quite popular.
Pulsing of a DC waveform for TIG welding can have either a low overall frequency of the pulses or high overall frequency. Low frequency pulsing with a pulsed output in the range of less than 20 Hz is used in many TIG welding applications. Generally pulsing at this low overall frequency tends to lower the heat input to the weld. In many applications, the user will set the pulsing frequency to the same rate as the filler metal is deposited in the weld puddle. The long low background current sections between the high current sections of the low frequency pulses cools the puddle and reduces the heat input to the welding process. This is a disadvantage of the common practice when using low frequency pulsing for TIG or GTAW welding. Consequently, high frequency pulsing has been attempted. The pulsed outputs for either DC positive or DC negative has an increased pulsing rate of 20-1,000 Hz and preferably in the general range of 60-500 Hz. Such high frequency is an effort to focus the arc onto the workpiece. They can not be obtained by standard transformer based power sources. So the high frequency technique has been made possible by the advent of inverter based power sources. Inverters or choppers can perform TIG welding process at high frequency. These devices have, thus, solved special requirements of particular TIG welding applications. Even though high frequency can be created when using inverters, DC current was still used for TIG welding. Furthermore, the DC current was pulsed at a low frequency, or occasionally, at a high frequency as explained.
Within the last twenty years, manufacturers have been offering GTAW machines with variable polarity capabilities. This is an AC waveform for TIG welding wherein the positive current section of the waveform and the negative current section of the waveform can be different in duration and/or amplitude. Early machines for creating this variable polarity output were constructed based upon SCR technology as shown in Risberg U.S. Pat. No. 4,038,515 and Stava U.S. Pat. No. 5,683,602. More recently, inverters have been employed for TIG welding, such as shown in Mita U.S. Pat. No. 5,225,660 and Stava U.S. Pat. No. 4,861,965. An inverter based power source produces a square current waveform. When using a waveform generator for controlling the pulse width modulator of the inverter, a variable polarity waveform can be produced where the positive peak current level is reduced without affecting the negative polarity waveform employed for the actual heating. As is known, with an AC power source, irrespective of the type of power source, the negative polarity portion of the AC waveform, is used for penetration and the positive polarity portion is used for cleaning the workpiece. By development of an inverter type power source for TIG welding, together with the use of waveform control employing a generator loaded with a desired profile from a look-up memory, a desired, stored state table defines the selected waveform profile. This advanced power source technology allows implementation of the present invention.